The invention relates in general to wireless communication networks and in particular to multiple input/multiple output (MIMO) modulation in a network with at least one degenerate node.
Wireless communication networks offer the possibility of convenient access to information from virtually anywhere. In one common model, a wireless local area network (LAN) is provided in which one or more potentially mobile units equipped with wireless network interface cards (NICs) exchange data with an access point (AP) via radio-frequency (RF) or infrared (IR) channels. The AP is connected—generally via a wired, high-speed connection—to a wide area network, intranet, LAN, the Internet, or other network. The NICs and APs use a common communication protocol to regulate the exchange of data. Generally speaking, at the physical level, the communication protocol specifies parameters for accessing the communication channel. Such parameters typically include the carrier frequencies and modulation formats to be used for transmitting data. In addition, the protocol typically provides for training data (i.e., known data sequences) to be transmitted; training data is used by the receiver to correct for channel effects that may lead to errors in reconstructing the transmitted data sequence, including fading and noise in the transmission channel, as well as carrier-frequency and timing offsets between the transmitter and the receiver and multipath effects.
The IEEE 802.11a standard defines one such protocol for data transmission in a wireless LAN using orthogonal frequency division multiplexing (OFDM) in the 5.8-GHz band. According to this standard, data to be transmitted is provided to the transmitter in serial form. Sequential data bits are mapped in groups to a symbol in a constellation; the choice of constellation depends on the desired data rate. To attain the maximum supported data rate of 54 Mbps, each group of six data bits is mapped to a symbol in a 64-point quadrature amplitude modulation (64QAM) constellation. Each symbol is then assigned to one of 48 mutually orthogonal data-carrying subcarrier frequencies; four additional orthogonal subcarrier frequencies are also used and carry pilot symbols. An Inverse Fast Fourier Transform (IFFT) may be used to combine symbol streams from all 52 subcarriers, producing a time-domain signal for transmission. A preamble is transmitted with each data frame. The preamble specifies the data rate (which determines the constellation to be used for decoding) and the length of the data in the frame. The preamble also includes training data needed for synchronization of the receiver with the transmitter and for estimation of channel effects. At the receiver end, the process is reversed. A Fast Fourier Transform (FFT) is performed, and channel effects are removed by using channel estimates generated from the training data. Based on the data rate provided in the preamble, the correct constellation is selected for decoding. Decoding generally involves estimating the most likely symbol sequence by mapping the received data (corrected for channel effects) to points in the constellation.
Compared with wired LANs, wireless LANs offer the advantages of low-cost installation and greater flexibility in relocating components, since it is not necessary to provide cables for connecting the components. However, the data rate in existing wireless LANs (e.g., a maximum of 54 Mbps in an IEEE 802.11a network) is considerably lower than the 100-Mbps rate available in a wired LAN using 100baseT Ethernet, for example. Thus, for networks where high data rates are needed, wired LANs are still preferred. To attain greater acceptance, protocols for wireless LANs that support higher data rates would be desirable.
Various options for IEEE 802.11a extensions that would support higher data rates have been considered. One option is to use multiple channels to transmit the data stream; for example, data could be multiplexed between two 54 Mbps streams occupying two contiguous 20 MHz channels to obtain a rate of 108 Mbps. Using multiple channels, however, would reduce the LAN capacity (i.e., the number of nodes) because each transmitter would require more of the available RF spectrum. Moreover, such schemes may not be permitted under existing telecommunication regulations in some areas (e.g., Europe).
Another option is to increase the constellation density and/or reduce the coding overhead, thereby enabling each symbol to carry more data bits. However, increased constellation density would lead to increased error rates at the receiver end, while reducing the coding overhead would reduce the error-correction information available to the receiver. In either case, the effective operating range of the transmitter would be reduced as compared to IEEE 802.11a specifications. Some of the range could be restored by using diversity techniques, in which replicas of the same signal, displaced in time and/or space, are sent or received. However, diversity techniques require additional antennas at one or both ends of the link, which increases the cost of manufacturing some or all of the network components. Some known techniques (e.g. maximal ratio combining, space-time coding, transform techniques, and adaptive beam-forming) are effective with additional antennas at only one end of the link and would reduce the extra cost associated with diversity techniques. Nevertheless, in order to double the data rate, the constellation density would have to be doubled, and diversity techniques would not fully compensate for the loss of range.
A third option is to use multiple input, multiple output (MIMO) systems to increase the data rate and the operating range without increasing bandwidth. In a MIMO system, a transmitter splits the data into multiple streams and sends each via a separate antenna. The transmitter is designed so that each transmitted data stream has a distinctive vector signature. The receiver, which also has multiple antennas, processes received signals as including components from multiple transmitted streams, based on the differences in vector signatures of the transmitted streams. Unfortunately, existing MIMO systems require all network components to have additional RF chains and antennas, which increases the cost.
Therefore, it would be desirable to provide a system that supported increased data rate and operating range while not requiring additional bandwidth or additional RF chains at every network node (i.e., component unit or AP) or requiring capability knowledge to be shared between every node.